Population-loss-induced narrowing of dark resonances

The dependence of the width of the coherent population trapping (CPT) reson ance on the atomic interaction time is studied theoretically. The optical B loch equations are solved for the Hanle/CPT schemes corresponding to the op en F-g-->F-e transitions of the sodium D-1 line interacting with linearly p olarized laser light. The mechanism producing the narrowing of the Hanle/CP T resonance is examined, the role of the population losses is pointed out, and the law describing the narrowing of the width of the resonance with the interaction time is determined. Our results are compared with the Hanle/CP T linewidth dependence on the interaction time occurring for a closed optic al transition. [S1050-2947(98)01012-9].